In no particular order:
1. Rotation Inhibitor, for those awkward viewing angles.
2. BossPaper, which allows for some basic theming (hide icon labels, format dock, wallpaper with rotate option), at a much lighter load than WinterBoard. Excellent for those of us who don't want to go crazy with the themes and subsequent lag, but are tired of the boring black and silver motif.
3. Flashlight. I know it seems silly, but why pay for a flashlight app that barely works? This one is much, much better as it adjusts brightness automatically.
4. MemTool, for advanced process viewing and a quick memory cleanup. I'm not sure how much it actually helps, but I've never had any trouble with it.
5. WeatherIcon plugin for LockInfo. Very nice to see weather on the lock screen.
Also, and one of my favorites (I forgot it because I see it so often it doesn't occur to me it's jailbreak-only): Lock Calendar. Lightweight, customizable, simple. Love, love, love it.